The synthesis of diamond crystals by high pressure, high temperature processes has become well established commercially. Preferred methods for making diamonds are disclosed and claimed in U.S. Pat. No. 2,947,610 to Hall et al and No. 2,947,609 to Strong. Apparatus for the conduct of such processes is described and claimed in U.S. Pat. No. 2,941,248 to Hall. The Hall et al, Strong and Hall patents are incorporated by reference.
Diamond growth in the aforementioned processes occurs by the diffusion of carbon through a thin metallic film of any of a series of specific catalyst-solvent materials. Although such processes are very successfully employed for the commercial production of industrial diamond, the ultimate crystal size of such diamond growth is limited by the fact that the carbon flux across the catalyst film is established by the solubility difference between graphite (the typical starting material) and the diamond being formed. This solubility difference is generally susceptible to significant decrease over any extended period due to a decrease in pressure in the system and/or poisoning effects in the graphite being converted.
On the other hand, in the method of growing diamond on a diamond seed crystal disclosed in U.S. Pat. No. 3,297,407 to Wentorf, Jr. (incorporated by reference) a difference in temperature between the diamond seed and the source of carbon is relied upon to establish a concentration gradient in carbon for deposition on the seed. Catalyst-solvents disclosed in the aforementioned Hall et al and Strong patents are used in the temperature gradient method as well. The growth of diamond on the seed material is driven by the difference in solubility of diamond in the molten catalyst-solvent metal at the nutrient (source of carbon) and at the seed, between which locations a temperature gradient exists. More important, this general type of reaction vessel configuration presents a pressure stable system so that pressure can more readily be kept in the diamond stable region.
By very carefully adjusting pressure and temperature and utilizing relatively small temperature gradients with extended (relative to growth times for thin film method) growth times larger diamonds can be produced by the method as taught in the Wentorf patent than by the thin-film method. However, attempts to increase crystal sizes to much greater than about 1/20 carat (by increasing the growth times by 5 to 10 times the aforementioned "extended" growth times) has shown that with these longer growth times the strong tendency for spontaneous nucleation of diamond crystals to occur at the underside of the molten catalyst-solvent metal develops into a serious problem, because that diamond nucleation occurring near the diamond seed material competes with the growth from the seed diamond resulting in the development of multiple crystals which collide as they grow and as a consequence produce stress fractures in the colliding crystals.